1. Field of the Invention:
This invention relates to a sputtering apparatus, and more particularly to a sputtering apparatus suitable for selectively depositing a film on the surface of an object to be processed in such a manner that the surface of the object is held in close contact with a mask.
2. Description of the Prior Art:
In recent years, a compact disc (hereinafter referred to as a CD) and a laser disc have been widely used, which can record a large amount of digitized sound and image information. The substrate of the CD is made of transparent synthetic resin such as polycarbonate and the like. A large number of so-called pits, which are minute concavo-convex portions, are formed on the surface of the substrate in accordance with digital information of "1" or "o". Further, an aluminum thin film layer having a high light-reflectivity is deposited on the pits-formed surface of the substrate by use of a sputtering technique. The thus recorded information can be read by the presence or absence of the reflected light of a laser light-beam applied to the CD. The aluminum thin film deposition per a single sheet of the disc substrates can be performed in a relatively short time. Thus, an apparatus shown in FIG. 6 has been employed in which the aluminum thin film can be continuously deposited on a large number of the disc substrates in a sheet-by-sheet manner.
FIG. 6 is a schematic diagram illustrating only the essential portions of a continuous sputtering apparatus. In FIG. 6, a large number of substrates 62 of CD and the like are sequentially transferred by use of an external transferring apparatus 61, such as a belt-conveyor and the like. The thus transferred substrates 62 are captured by suction pads 64 provided on a disc-shaped internal transferring apparatus 63, which is rotatable about its shaft and movable in a vertical direction, and then transferred into a transferring chamber 65. In the transferring chamber 65, substrates 62 are placed on a transferring table 66 which is rotatable about its shaft and movable in a vertical direction. The thus placed substrates 62 are sputtered by use of a sputtering source 67 in a prescribed sheet-by-sheet manner. After the completion of sputtering, the substrates 62 are sequentially taken out by use of the transferring table 66 and the internal transferring apparatus 63.
FIG. 7 is an enlarged cross-sectional view illustrating the vicinity of the sputtering source 67. In FIG. 7, a mask 69 is provided under an aluminum target 68 which is used in the sputtering source 67. The substrate 62 is pushed up against the mask 69 by the transferring table 66 so as to make close contact with the mask 69. A film-deposition chamber 70, in which the sputtering for film deposition is performed, is constituted by a space surrounded with an envelope 71, the aluminum target 68 and the mask 69. The exhaust of the film-deposition chamber 70 is performed, while the substrate 62 is not in close contact with the mask 69, by use of a turbo-molecular pump (not shown) through an opening 69a of the mask 69 and an exhaust pipe 72 connected to the transferring chamber 65. Assume that the substrate 62 is in close contact with the mask 69 for the film-deposition process. In this case, the surface of substrate 62 is partially exposed or masked with respect to the aluminum target 68. Specifically, an aluminum film is deposited only on the portion of the substrate 62 corresponding to the opening 69a of the mask 69. The aluminum film is not deposited on the portion of the substrate 82 masked by the mask 69. The mask 69 is manufactured with high accuracy, and fixed to the envelope 71 at its opening 74 so as to be in close contact therewith by use of a thermal-expansion force of the mask 69. In the process of sputtering for film deposition, argon gas is introduced into the film-deposition chamber 70 through a gas-supplying pipe 73. Further, a voltage is applied to the aluminum target 68 through an electrode (not shown), and the target 68 is bombarded by the separated argon atoms. Thus, aluminum atoms are freed from the target 68, and then deposited on the surface of substrate 62 at the portion which has not been masked by the mask 69. As a result, the aluminum thin film layer can be deposited on the surface of substrate 62. After the completion of the film-deposition process, the discharge is stopped, and the substrate 62 is transferred from the transferring table 66 to the internal transferring apparatus 63, and then transferred to the outside.
In the above-described sputtering apparatus, the processes of transfer and film-deposition per a sheet of substrates 62 are performed in a short period of about 6 seconds. The actual film-deposition process is performed in an extremely short period of about 2 seconds in which the electric field is being applied to the target 68. Assume that the gas is continuously supplied through the gas-supplying pipe 73 to the film-deposition chamber 70 which is hermetically sealed by the mask 69 and the substrate 62 and has no exhaust holes. In this case, the pressure in the film-deposition chamber 70 is continuously increased even in a short period of about 2 seconds for the film-deposition process. Thus, the discharge current-voltage characteristics in the film-deposition process are inevitably changed. As a result, the stable film-depositing conditions cannot be assuredly obtained. In order to stabilize the pressure in the film-deposition chamber 70, a plurality of exhaust holes 75 are provided in the envelope 71 at the positions adjacent to the mask 69, as shown in FIG. 7. The exhaust holes 75 serve to by-pass the pressure in the film-deposition chamber 70 to the transferring chamber 65. However, aluminum for use in the film-deposition sputtering process inevitably flows into the transferring chamber 65 through the exhaust holes 75. As a result, such aluminum contaminates the transferring chamber 65. Therefore, it is not appropriate countermeasure to provide the exhaust holes 75 in the boundary wall between the film-deposition chamber 70 and the transferring chamber 65.
As described above, in the conventional sputtering apparatus, the pressure in the film-deposition chamber 70 cannot be stabilized without the exhaust holes 75. However, when the exhaust holes 75 are provided for by-passing the pressure in the film-deposition chamber 70, the sputtering material, i.e., aluminum inevitably contaminates the inside of the exhaust holes 75 and the transferring chamber 65. Thus, the satisfactory sputtering conditions for the prescribed film-deposition process cannot be easily maintained.